1. Field of the Invention
The present invention relates to a photocathode, a phototube, and a photodetecting apparatus and, more particularly, to a photodetecting technique for obtaining one- or two-dimensional information, e.g., an incident position or an incident light image of weak light.
2. Related Background Art
To perform photodetection including detection of one- or two-dimensional position information of weak light, an apparatus constituted by an image intensifier combined with a solid-state image sensor is generally used. In this apparatus, photoelectrons are excited by photons which are incident from the input window of a housing on a photocathode. The photoelectrons emitted from the photocathode into a vacuum are focused and accelerated by an electron lens system. Thereafter, the photoelectrons are focused by a phosphor and converted into an optical signal again, thereby intensifying the light. Photoelectric conversion of this intensified optical signal is performed by the solid-state image sensor, such as a CCD, and position information is extracted as an electrical signal.
A photomultiplier having a position detecting function is also used for photodetection. In this apparatus, the anode of the photomultiplier is divided and multiplied to perform photodetection, thereby obtaining position information. In addition, another example of a photomultiplier having the position detecting function is described in Japanese Patent Laid-Open No. 60-20441. In this photomultiplier, a photocathode is formed on the inner wall of a faceplate. A mesh electrode is provided between the photocathode and a focusing electrode for forming an electric field which guides photoelectrons emitted from the photocathode to a first-stage dynode. This mesh electrode is arranged on only one side at a position away from the photocathode by a 1/10 distance between the photocathode and the focusing electrode. The mesh electrode forms a field distribution for gradually preventing the photoelectrons from reaching the first-stage dynode from one side to the other side. Of the photoelectrons emitted from the entire photoelectron emission surface of the photocathode, photoelectrons on one side are prevented from reaching the first-stage dynode when a bias voltage is applied to the mesh electrode. More specifically, the orbits of the photoelectrons are changed to multiply only photoelectrons emitted from a predetermined portion of the emission surface and output them as an electrical signal. On the basis of the output signal level and the bias voltage level applied to the mesh electrode, photodetection with position resolution is performed by an external determination apparatus. In this manner, only the photoelectrons which are excited by light incident on a specific position and whose orbits are not interrupted are detected to perform position detection.
In the conventional apparatus in which an image intensifier and a solid-state image sensor are combined, conversion of optical signal.fwdarw.electrical signal.fwdarw.optical signal.fwdarw.electrical signal cannot be substantially avoided. Therefore, a coupling loss or the like decreases the efficiency, resulting in poor performance.
In the multianode photomultiplier, crosstalk between the photocathode and the multiplier section, or between the multiplier section and the anode poses a problem, and the position resolution is not substantially improved.
In the photomultiplier having a mesh electrode, only some of photoelectrons emitted from the entire photoelectron emission surface of the photocathode are detected upon measurement to perform position detection. For this reason, a substantial problem on S/N ratio arises. As for the position resolution, the orbits of the photoelectrons are changed to perform position determination, the crosstalk is structurally increased. In addition, position determination is possible at only about two portions for one photomultiplier, and it is substantially difficult to realize a multi-element structure.